The present invention relates to the selective magnetic resonance examination of moving material. It finds particular application in conjunction with the magnetic resonance imaging of separately identified venous and arterial blood flows and will be described with particular reference thereto. It is to be appreciated, however, that the present invention may also find application in conjunction with the imaging of other flowing, accelerating, or moving tissue or materials as well as spectroscopic analysis of such moving materials.
Heretofore, magnetic resonance angiography has had its basis in three main features: (1) the tagging or sensitizing of an NMR signal to flow, (2) the suppression or removal of signal from static material, or (3) projection imaging. Of the several methods by which the magnetic resonance signal can be sensitized or tagged to flow, two are most common. The first depends on the time-of-flight effect which depends on a wash-in/wash-out phenomenon. The second, gradient moment nulling, depends upon flow-dependent phase shifts.
The time-of-flight effects are used in two ways. In one, blood which is flowing into the imaging region is tagged just before an image is collected. When an image is acquired, the tagged blood that has flowed into the image region gives a characteristic signal that can be used to separate it from the stationary material. In a second way, a special RF pulse is used during each repetition to label the moving blood. The signal or lack of signal from the labelled blood can be detected when the tagged blood has moved into the imaging volume.
Flow dependent phase shifts can also be utilized for flow signal sensitizing. By selecting the gradients to zero selected orders of gradient moments, flow or other motion can be tagged with phase information. Quantitative direction, speed, and other information can be derived from the phase. See for example, U.S. Pat. Nos. 4,689,560 and 4,683,431, both to G. L. Naylor and P. M. Pattany. Analogously, phase can be nulled or zeroed for a number of gradient moments to maximize the flow signal. See U.S. Pat. No. 4,728,890 to Pattany and McNally.
The suppression or removal of the static material may be achieved by several techniques. For flow related enhancement techniques, a ray tracing algorithm can be utilized to pick out and isolate the highest signal in the image set. With blood tagged to have the highest signal, these identified regions correlate to the moving blood.
The static signal may also be removed by subtracting two images. That is, a first image is taken utilizing one of the above techniques to tag the flow and a second image is taken without flow encoding. When the static image is subtracted from the flow sensitized image, only the moving tissue remains. Note again, the above referenced patents of Naylor and Pattany.
In angiography, it is often advantageous to differentiate between venous and arterial flows. Conveniently, the venous and arterial flows are usually in opposite directions through the selected slice or other imaging region. To image both flows individually, three image sets are taken. For example, one of the time-offlight effects may be utilized to encode the blood entering from one side of the imaging region just before data for a first image is acquired. Blood entering from the opposite side of the image region is analogously encoded just before for a second image is acquired. A third image of the static material without flow encoding is also acquired, either before, after, or between the two flow images. Subtracting the static image from one of the flow images provides a venogram and subtracting the static image from the other flow image provides an arteriogram. Thus, three sets of image data are required in order to generate two flow images. See for example, U.S. Pat. No. 4,849,697 to Cline and Dumoulin.
A third factor associated with magnetic resonance angiography is image projection. Some of these methods attempt to capture vessels in very thick, directly acquired projection images, while others use postprocessing to piece together stacks of slices into a three dimensional data set. The slices may have been acquired either as multiple two dimensional slices or as part of a three dimensional volume acquisition.
A primary problem with these flow imaging techniques is that they required the acquisition of enough data to reconstruct three images of the same slice or region in order to produce only two angiogram images. Further, when these techniques were implemented such that the flowing material was tagged due to an applied gradient phase shift, it was subject to error due to magnetic imperfections, e.g. eddy currents. These imperfections introduced unwanted and uncompensated phase shifts which were erroneously treated as properly phase encoded information. Others of these techniques required a compromise between the flow to stationary material contrast. That is, the imaging pulse sequences were chosen, as best as possible, such that the flowing material appeared bright, while the static material appeared low in intensity and somewhat iso-intense.
The present invention provides a new and improved imaging technique which overcomes the above referenced problems and others.